Cyclic nucleotide phosphodiesterases (PDEs) that catalyze the hydrolysis of 3'5' cyclic nucleotides such as cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) to the corresponding nucleoside 5' monophosphates constitute a complex family of enzymes. By mediating the intracellular concentration of the cyclic nucleotides, the PDE isoenzymes function in signal transduction pathways involving cyclic nucleotide second messengers.
A variety of PDEs have been isolated from different tissue sources and many of the PDEs characterized to date exhibit differences in biological properties including physicochemical properties, substrate specificity, sensitivity to inhibitors, immunological reactivity and mode of regulation. [See Beavo et al., Cyclic Nucleotide Phosphodiesterases: Structure, Regulation and Drug Action, John Wiley & Sons, Chichester, U.K. (1990)] Comparison of the known amino acid sequences of various PDEs indicates that most PDEs are chimeric multidomain proteins that have distinct catalytic and regulatory domains. [See Charbonneau, pp. 267-296 in Beavo et al., supra] All mammalian PDEs characterized to date share a sequence of approximately 250 amino acid residues in length that appears to comprise the catalytic site and is located in the carboxyl terminal region of the enzyme. PDE domains that interact with allosteric or regulatory molecules are thought to be located within the amino-terminal regions of the isoenzymes. Based on their biological properties, the PDEs may be classified into six general families: the Ca.sup.2+ /calmodulin-stimulated PDEs (Type I), the cGMP-stimulated PDEs (Type II), the cGMP-inhibited PDEs (Type III), the cAMP-specfic PDEs (Type IV), the cGMP-specific phosphodiesterase cGB-PDE (Type V) which is the subject of the present invention and the cGMP-specific photoreceptor PDEs (Type VI).
The cGMP-binding PDEs (Type II, Type V and Type VI PDEs), in addition to having a homologous catalytic domain near their carboxyl terminus, have a second conserved sequence which is located closer to their amino terminus and which may comprise an allosteric cGMP-binding domain. See Charbonneau et al., Proc. Natl. Acad. Sci. USA, 87: 288-292 (1990).
The Type II cGMP-stimulated PDEs (cGs-PDEs) are widely distributed in different tissue types and are thought to exist as homodimers of 100-105 kDa subunits. The cGs-PDEs respond under physiological conditions to elevated cGMP concentrations by increasing the rate of cAMP hydrolysis. The amino acid sequence of a bovine heart cGs-PDE and a partial cDNA sequence of a bovine adrenal cortex cGS-PDE are reported in LeTrong et al., Biochemistry, 29: 10280-10288 (1990) and full length bovine adrenal and human fetal brain cGS-PDE cDNA sequences are described in Patent Cooperation Treaty International Publication No. WO 92/18541 published on Oct. 29, 1992. The full length bovine adrenal cDNA sequence is also described in Sonnenburg et al., J. Biol. Chem., 266: 17655-17661 (1991).
The photoreceptor PDEs and the cGB-PDE have been described as cGMP-specific PDEs because they exhibit a 50-fold or greater selectivity for hydrolyzing cGMP over cAMP.
The photoreceptor PDEs are the rod outer segment PDE (ROS-PDE) and the cone PDE (COS-PDE). The holoenzyme structure of the ROS-PDE consists of two large subunits at (88 kDa) and .beta. (84 kDa) which are both catalytically active and two smaller .gamma. regulatory subunits (both 11 kDa). A soluble form of the ROS-PDE has also been identified which includes .alpha.,.beta., and .gamma. subunits and .delta. subunit (15 kDa) that appears to be identical to the COS-PDE 15 kDa subunit. A full-length cDNA corresponding to the bovine membrane-associated ROS-PDE .alpha. subunit is described in Ovchinnikov et al., FEBS Lett., 223: 169-173 (1987) and a full length cDNA corresponding to the bovine rod outer segment PDE .beta. subunit is described in Lipkin et al., J. Biol. Chem., 265: 12955-12959 (1990). Ovchinnikov et al., FEBS Lett., 204: 169-173 (1986) presents a full-length cDNA corresponding to the bovine ROS-PDE .gamma. subunit and the amino acid sequence of the .delta. subunit. Expression of the ROS-PDE has also been reported in brain in Collins et al., Genomics, 13: 698-704 (1992). The COS-PDE is composed of two identical .alpha.' (94 kDa) subunits and three smaller subunits of 11 kDa, 13 kDa and 15 kDa. A full-length cDNA corresponding to the bovine COS-PDE .alpha.' subunit is reported in Li et al., Proc. Natl. Acad. Sci. USA, 87: 293-297 (1990).
cGB-PDE has been purified to homogeneity from rat [Francis et al., Methods Enzymol., 159: 722-729 (1988)] and bovine lung tissue [Thomas et al., J. Biol. Chem., 265: 14964-14970 (1990), hereinafter "Thomas I"]. The presence of this or similar enzymes has been reported in a variety of tissues and species including rat and human platelets [Hamer et al., Adv. Cyclic Nucleotide Protein Phosphorylation Res., 16: 119-136 (1984)], rat spleen [Coquil et al., Biochem. Biophys. Res. Commun., 127: 226-231 (1985)], guinea pig lung [Davis et al., J. Biol. Chem., 252: 4078-4084 (1977)], vascular smooth muscle [Coquil et al., Biochem. Biophys. Acta, 631: 148-165 (1980)], and sea urchin sperm [Francis et al., J. Biol. Chem., 255: 620-626 (1979)]. cGB-PDE may be a homodimer comprised of two 93 kDa subunits. [See Thomas I, supra] cGB-PDE has been shown to contain a single site not found in other known cGMP-binding PDEs which is phosphorylated by cGMP-dependent protein kinase (cGK) and, with a lower affinity, by cAMP-dependent protein kinase (cAK). [See Thomas et al., J. Biol. Chem., 265: 14971-14978 (1990), hereinafter "Thomas II"] The primary amino acid sequence of the phosphorylation site and of the amino-terminal end of a fragment generated by chymotryptic digestion of cGB-PDE are described in Thomas II, supra, and Thomas I, supra, respectively. However, the majority of the amino acid sequence of cGB-PDE has not previously been described.
Various inhibitors of different types of PDEs have been described in the literature. Two inhibitors that exhibit some specificity for Type V PDEs are zaprinast and dipyridamole. See Francis et at., pp. 117-140 in Beavo et al., supra.
Elucidation of the DNA and amino acid sequences encoding the cGB-PDE and production of cGB-PDE polypeptide by recombinant methods would provide information and material to allow the identification of novel agents that selectively modulate the activity of the cGB-PDEs. The recognition that there are distinct types or families of PDE isoenzymes and that different tissues express different complements of PDEs has led to an interest in the development of PDE modulators which may have therapeutic indications for disease states that involve signal transduction pathways utilizing cyclic nucleotides as second messengers. Various selective and non-selective inhibitors of PDE activity are discussed in Murray et at., Biochem. Soc. Trans., 20(2): 460-464 (1992). Development of PDE modulators without the ability to produce a specific PDE by recombinant DNA techniques is difficult because all PDEs catalyze the same basic reaction, have overlapping substrate specificities and occur only in trace amounts. As a result, purification to homogeneity of many PDEs is a tedious and difficult process.
There thus continues to exist a need in the art for DNA and amino acid sequence information for the cGB-PDE, for methods and materials for the recombinant production of cGB-PDE polypeptides and for methods for identifying specific modulators of cGB-PDE activity.